Plasma processes offer the opportunity to make coatings that can be quite hard, chemically inert, corrosion resistant, and impervious to water vapor and oxygen. These are often used as mechanical and chemical protective coatings on a wide variety of substrates. For example, carbon-rich coatings (e.g., diamond-like carbon and jet plasma carbon coatings) have been applied to rigid disks and flexible magnetic media. They have also been applied to acoustic diaphragms, polymeric substrates used in optical and ophthalmic lenses, as well as electrostatic photographic drums. Silicon-containing polymer coatings have been applied to polymeric and metal substrates for abrasion resistance. Also, silicone coatings have been applied to polymeric and nonpolymeric substrates to reduce water permeability and to provide mechanical protection.
Carbon-rich coatings, as used herein, contain at least 50 atom percent carbon, and typically about 70-95 atom percent carbon, 0.1-20 atom percent nitrogen, 0.1-15 atom percent oxygen, and 0.1-40 atom percent hydrogen. Such carbon-rich coatings can be classified as "amorphous" carbon coatings, "hydrogenated amorphous" carbon coatings, "graphitic" coatings, "i-carbon" coatings, "diamond-like" coatings, etc., depending on their physical and chemical properties. Although the molecular structures of each of these coating types are not always readily distinguished, they typically contain two types of carbon-carbon bonds, i.e., trigonal graphite bonds (sp.sup.2) and tetrahedral diamond bonds (sp.sup.3), although this is not meant to be limiting. They can also contain carbon-hydrogen bonds and carbon-oxygen bonds, etc. Depending on the amount of noncarbon atoms and the ratio of sp.sup.3 /sp.sup.2 bonds, different structural and physical characteristics can be obtained.
Diamond-like carbon-rich coatings have diamond-like properties of extreme hardness, extremely low electrical conductivity, low coefficients of friction, and optical transparency over a wide range of wavelengths. They can be hydrogenated or nonhydrogenated. Diamond-like carbon coatings typically contain noncrystalline material having both trigonal graphite bonds (sp.sup.2) and tetrahedral diamond bonds (sp.sup.3); although it is believed the sp.sup.3 bonding dominates. Generally, diamond-like coatings are harder than graphitic carbon coatings, which are harder than carbon coatings having a large hydrogen content, i.e., coatings containing hydrocarbon molecules or portions thereof.
Silicon-containing coatings are usually polymeric coatings that contain in random composition silicon, carbon, hydrogen, oxygen, and nitrogen (SiO.sub.w N.sub.x C.sub.y H.sub.z). These coatings are usually produced by plasma enhanced chemical vapor deposition (PECVD) and are useful as barrier and protective coatings. See, for example, U.S. Pat. Nos. 5,298,587 (Hu et al.), 5,320,875 (Hu et al.), 4,830,873 (Benz et al.), and 4,557,946 (Sacher et al.).
Silicone coatings are high molecular weight polymerized siloxane coatings containing in their structural unit R.sub.2 SiO in which R is usually CH.sub.3 but may be H, C.sub.2 H.sub.5, C.sub.6 H.sub.5, or more complex substituents. These silicones (often referred to as polyorganosiloxanes) consist of chains of alternating silicon and oxygen atoms (O--Si--O--Si--O) with the free valences of the silicon atoms joined usually to R groups, but also to some extent to oxygen atoms that are joined to (crosslinked) silicon atoms in a second chain, thereby forming an extended network. These coatings are valued for their toughness, their lubricity, controlled gas diffusion, and their ability to lower surface tension desirable for release coatings and water repellent surfaces. For example, U.S. Pat. No. 5,096,738 (Wyman) teaches the formation of barrier coatings via the hydrolysis of trialkoxy methyl silane resulting in highly crosslinked polymer structures.
Methods for preparing coatings by plasma deposition, i.e., plasma-enhanced chemical vapor deposition, are known; however, some of these methods have deficiencies. For example, with certain methods the use of high gas flow, pressure, and power can cause formation of carbon powder, instead of the desirable smooth, hard carbon film. U.S. Pat. Nos. 5,232,791 (Kohler et al.), 5,286,534 (Kohler et al.), and 5,464,667 (Kohler et al.) disclose a process for the plasma deposition of a carbon-rich coating that overcomes some of these deficiencies. These processes use a carbon-rich plasma, which is generated from a gas, such as methane, ethylene, methyliodide, methylcyanide, or tetramethylsilane, in an elongated hollow cathode, for example. The plasma is accelerated toward a substrate exposed to a radio frequency bias voltage. Although this process represents a significant advancement in the art, other plasma deposition processes are needed for deposition of a wide variety of carbon- and/or silicon-containing coatings using lower energy requirements.
Methods of preparing multilayer coatings are described in U.S. Pat. Nos. 5,116,665 (Gauthier et al.) and 4,933,300 (Koinuma et al.), and UK Patent Application Publication No. GB 2 225 344 A (Eniricerche SpA). These methods are based on glow discharge processes, which utilize one reactor and successive changes in process parameters for the construction of multilayer coatings. These methods, however, have practical and technical limitations. A batch type process is required if gradual and/or abrupt changes of layer properties are desired. Those changes are obtained by deposition on stationary substrates and successive changes in process conditions. Continuous deposition can be obtained in a reactor that accommodates a roll-to-roll web transport system. Multipass operation is required to construct multilayer coatings. Under those circumstances a gradual change of layer properties and/or the formation of interfacial layers are difficult to obtain.
Thus, plasma deposition processes are needed for deposition of a wide variety of carbon- and/or silicon-containing coatings using relatively low energy requirements. Also, plasma deposition processes are needed that can accommodate a gradual change of layer properties and/or the formation of interfacial layers.